In recent years, there has been wide spread use of induction heating cookers for inductively heating cooking containers such as pans, frying pans and the like, using heating coils.
Conventionally, among induction heating cookers of this type, there have been known induction heating cookers employing infrared-ray detection means for detecting infrared rays radiated from cooking containers according to the temperatures thereof and for outputting infrared-ray detection signals according to the detected infrared-ray energy, in order to detect the temperatures of the cooking containers with higher accuracy.
Further, Japanese Patent No. 4311154 (Patent Literature 1) has suggested a structure which is adapted to detect the temperature of a cooking container when the cooking container is at a lower temperature (70° C.) using an infrared-ray sensor as infrared-ray detection means, and is adapted to control heating based on the detected temperature.
The infrared-ray detection means used in the induction heating cooker raises its temperature by being subjected to heat radiated from the cooking container being heated, a top plate on which the cooking container is placed, a heating coil for induction heating, and the like. In cases where the infrared-ray detection means includes a photo diode which is quantum-type infrared-ray reception means, and an operational amplifier for performing current-to-voltage conversion on electric current signals outputted from the photo diode and for amplifying the signals, if the temperature of the photo diode is raised, this lowers the resistance value of the parallel resistance (the shunt resistance) which is the internal resistance in the photo diode. If the resistance value of the parallel resistance is lowered as described above, the input offset voltage in the operational amplifier is amplified to be increased.
As a result, the amplified input offset voltage is superimposed on the infrared-ray detection signal outputted from the infrared-ray detection means, which induces the problem that the infrared-ray detection signal outputted from the infrared-ray detection means can not accurately indicate the infrared-ray energy. In order to overcome this problem for preventing degradation of the accuracy of detection of the temperature of the cooking container through the infrared-ray detection signal, JP-A No. 2008-52959 (Patent Literature 2) has suggested an induction heating cooker provided with connection control means for periodically reversing the polarity of the photocurrent outputted from a photo diode.
Ordinary induction heating cookers have been adapted to detect the temperature of a pan bottom of a cooking container and to control heating of the cooking container, using infrared-ray detection means provided under a top plate.
In an induction heating cooker, the top plate is made of a heat-resistant glass having a light transmittance of about 90% (in the case where its thickness is 4 mm) for a wavelength range of 0.5 to 2.5 μm, and the infrared-ray detection means detects infrared rays within this wavelength range. Referring to FIG. 1, a solid line indicates a characteristic curve representing a light-transmittance characteristic of a heat-resistant glass which is generally used in a top plate. Further, in FIG. 1, respective broken lines represent radiant energy from blackbodies at certain temperatures (60° C. and 140° C.). Further, there are illustrated, by hatching, the areas of radiant energy which can be received by infrared-ray detection portions, which will be described later.
Further, referring to FIG. 1, the lateral axis represents the wavelength [micrometer], while the longitudinal axis represents the light transmittance [%] and the radiation intensity [W/sr]. Here, the light transmittance, which is a value indicating the degrees of light absorbance and light reflection, represents the ratio of the amount of light which is penetrated and emitted, to the amount of incident light.
JP-A No. 2009-176553 (Patent Literature 3) has suggested an induction heating cooker employing an infrared-ray sensor as infrared-ray detection means for detecting a certain temperature range, by identifying a detection range of received infrared rays. In the induction heating cooker, the infrared-ray sensor is provided with a hemispherical lens made of a polycarbonate, in order to condense infrared rays. Since the lens is made of a resin, it is possible to reduce the cost of the infrared-ray detection means.    PLT 1: Japanese Patent No. 4311154    PLT 2: JP-A No. 2008-52959    PLT 3: JP-A No. 2009-176553